1. Field of the Invention
The present invention relates to a carbon monoxide (CO) conversion catalyst (CO shift catalyst) for removing carbon monoxide from hydrogen-containing gas, which is a reformer catalyst for PEFC used in a reformer for producing hydrogen, a method for producing a CO conversion catalyst, and a fuel cell system.
2. Description of the Related Art
A polymer electrolyte fuel cell (PEFC) is a low-pollution cell and has a high thermal efficiency, and hence is recently expected to be applied to a power source in a wide variety of fields, such as a power source for automobile and a dispersed power system. In this fuel cell system, hydrogen (H2) is produced by reforming a hydrocarbon fuel (such as city gas, methane, propane, kerosine, or dimethyl ether) by a reformer (see (I) reforming reaction shown below).
The reformed gas obtained by reforming using a reformer contains carbon monoxide (CO) and carbon dioxide (CO2) in addition to hydrogen (H2), and carbon monoxide (CO) poisons platinum which is mainly used as an electrocatalyst for the fuel cell. Therefore, a method for reducing the concentration of carbon monoxide (CO) in the gas obtained by reforming is employed in which the method includes a CO shift reaction performed in a CO conversion device using a CO conversion catalyst and a CO oxidation reaction performed in a CO removing device using a CO removing catalyst (see (II) CO shift reaction and (III) CO partial oxidation reaction shown below).
I) Reforming ReactionCH4+H2O→3H2+CO  (1)CH4+½O2→2H2+CO  (2)II) CO Shift ReactionCO+H2O→CO2+H2  (3)III) CO Partial Oxidation Reaction (CO Removal Reaction)CO+½O2→CO2  (4)H2+½O2→H2O  (5)
For steadily reducing the carbon monoxide (CO) contained in the reformed gas obtained by reforming using a reformer, the CO conversion catalyst is required to have high selectivity and high reactivity. Therefore, a noble metal catalyst including platinum (Pt) has been used as a CO conversion catalyst (see Japanese Patent Application Laid-open No. 2005-131471 and Japanese Patent Application Laid-open No. 2004-216369).
The noble metal catalyst including platinum (Pt) has such high activity that it can steadily reduce the carbon monoxide (CO) concentration. However, the noble metal catalyst is expensive and hence increases the cost of producing CO conversion catalysts, making it difficult to reduce the cost for polymer electrolyte fuel cell (PEFC) system.
To solve the above problem, a copper (Cu) catalyst, such as a Cu/Zn catalyst, has been proposed (see Japanese Patent Application Laid-open No. 2006-252929).
The copper catalyst is inexpensive as compared to a noble metal catalyst, however, the copper catalyst has a problem that it is adversely affected by steam or the like and it is poor in activity and durability.
Particularly in a daily startup and shutdown (DSS) operation in which startup and shutdown are performed frequently, a problem arises that the activity of the CO conversion catalyst is remarkably lowered.
FIG. 18 is a schematic diagram for explaining a change of a state of water as the temperature of a catalyst layer is increased or lowered. As shown in FIG. 18, for example, in a DSS operation, water (H2O) at about 50° C. is in a liquid state. After the start of operation, the temperature of the catalyst layer is increased to, for example, 200° C. With respect to the Cu catalyst, such as Cu/Zn catalyst, when the temperature of the catalyst layer has reached, for example, about 100° C., copper (Cu) constituting the Cu/Zn catalyst undergoes a steam oxidation represented by the formula (6) below, causing sintering of copper (Cu).Cu+H2O→Cu(OH)2→CuO→Cu  (6)
When the temperature of the catalyst layer has reached, for example, about 200° C. and then the operation is stopped and the temperature is lowered to 50° C., for example, the CO conversion device is shut off from the outside and a temperature difference of about 150 to 200° C. is caused, so that a change in a volume of a gas is caused in the CO conversion device. As a result, air in the external atmosphere goes into the CO conversion device, thus forming a mixture of air and water in the CO conversion device.
As described above, a conventional Cu/Zn catalyst has the following problem. As the temperature is increased and lowered repeatedly, as shown in FIG. 19, vaporization of liquid water causes oxidation, and the reformed gas causes reduction of copper oxide (CuO) and heat generation, so that sintering of copper occurs to cause grain growth, lowering the catalyst in activity.
Therefore, there has been strongly desired the development of a CO conversion catalyst for fuel cell, which catalyst uses an inexpensive copper catalyst and has high durability, and is advantageous in that sintering of copper can be prevented even when the catalyst is used in a fuel cell in a DSS operation.